1. Field of the Invention
The present invention relates to a layered film including a heteroepitaxial PN junction oxide thin film.
2. Description of the Related Art
PN junctions formed by elemental semiconductors such as Si and Ge and compound semiconductors such as GaAs, InP, and GaN are widely used in solid-state electronic devices such as bipolar transistors and rectifier diode devices and solid-state photoelectronic devices such as semiconductor lasers, light-emitting diodes, photodetectors, and solar cells. Homogeneous PN junctions composed of the same types of compounds are often used in these devices. However, in semiconductor lasers and light-emitting diodes, heterogeneous PN junctions composed of compounds of different types are used.
In order to ensure optimum device characteristics and reproducibility thereof for these electronic devices, the crystallinity of functional films is desirably high. It is difficult to obtain favorable device characteristics from polycrystalline substances due to disturbance of physical quantities at grain boundaries. Accordingly, epitaxial films as close to single crystals as possible are needed.
However, as in the case of making heterogeneous PN junctions, it is difficult with a crystal growth technique to join semiconductors of different types at the interface at which rapid atomic-layer-level compositional changes occur and to conduct deposition while maintaining high crystallinity of the semiconductor layers. Furthermore, materials used for forming PN junctions, in particular, compound semiconductor materials, are in many cases thermally and chemically unstable, hazardous to environment, and facing depletion of resources. Some of the challenges imposed on PN junction devices that use semiconductor materials can be addressed by using oxide semiconductor materials.
However, it is still difficult to form semiconductor thin films with oxides, in particular, to form a P-type semiconductor with an oxide. Japanese Unexamined Patent Application Publication No. 2004-119525 discloses a PN junction device and reports that a PN junction having a heteroepitaxial interface is realized by depositing a polycrystal P-type oxide film on an epitaxially grown N-type semiconductor oxide thin film and then annealing the two films. However, annealing a film deposited to be in a polycrystal state will only improve the crystallinity of each of the domains constituting the polycrystal state and will not give an epitaxial film that is close to single crystal. There has been no report of forming a heteroepitaxial PN junction with oxide semiconductor thin films.
In order to achieve good PN junction device characteristics from oxide semiconductors, epitaxial films that are close to single crystal as much as possible are needed. In particular, in order to stack materials of different types as in forming a heteroepitaxial PN junction, materials having different lattice constants must be stacked while maintaining the crystallinity. Accordingly, there is a high expectation for development of a technique for c-axis-oriented epitaxial growth of oxide semiconductors.